A thin and uniform light source device low in power consumption and high in luminance must be disposed for a light source device for a liquid crystal display device or the like used for a portable information terminal, a notebook computer, a liquid crystal television or the like. Especially, in a portable electronic device such as potable telephone or a portable information terminal having a relatively compact liquid crystal display device, such a requirement is stronger.
Conventionally, as a system of a back surface light source device used for a liquid crystal display device, a billboard, a traffic guide board or the like, there are a direct-under lighting system including a plurality of linear light sources such as fluorescent lamps installed in a housing, and an edge lighting system including a line light source arranged in a side end face of a plate-like light guide. The back surface light source device of the direct-under lighting system has had a difficulty of weight-reducing or thinning of a light source portion, and a problem of easy occurrence of a see-through phenomenon where the fluorescent lamp or the like used as a primary light source is seen through from a sign board. As a light and thin back surface light source device, the edge lighting system has frequently been used. In recent years, there has also been an increase in demand for a mobile electronic device such as a portable telephone, an electronic notebook, or a game machine. Thus, there is a desire for development of a thin back surface light source device having high luminance and good uniformity of a luminance distribution to be used as the light source portion for a display unit of such a device.
In the case of the back surface light source device of the edge lighting system, normally, the light guide is made of a plate-like transparent material such as an acrylic resin plate, a light from a primary light source (may be simply referred to as “light source”, hereinafter) arranged to face its side end face is made incident on the light guide from the side end face (light input surface or light incident face), and the incident light is outputted or emitted from a light output surface or light emitting surface by providing a light output or emission function unit such as a light scattering or diffusing surface or the like formed in a front surface (light output surface) of the light guide or a back surface opposite the front surface. However, in the device including the light output function unit uniformly formed in the front surface or the back surface of the light guide, luminance of the outputted light is reduced as it is more apart from the light source. Consequently, luminance in the light output surface becomes non-uniform, making it impossible to obtain a good display screen. Such a tendency becomes conspicuous as the surface light source device is enlarged, and cannot endure practical use in a surface light source device of 10 inches or more. Especially, in the case of the liquid crystal display device used for a notebook computer, a liquid crystal television or the like, very high uniformity of a luminance distribution in its screen is required.
Various presentations have been made in order to solve the problem of the non-uniform luminance of the surface light source device. For example, JP(A)-1-24522 presents a surface light source device including a light output function unit in a back surface opposite a light output surface of a light guide, on which a light diffusion material is applied or deposited densely as it is more apart from a light input surface. JP(A)-1-107406 presents a surface light source device which uses a light guide obtained by laminating a plurality of transparent plates each having small spots made of light scattering materials formed in various patterns on a surface. In such surface light source devices, there is a drawback that use of a white pigment of titanium oxide, barium sulfate or the like as a light scattering material causes a light loss such as light absorption when a light incident on the light scattering material is scattered, leading to a reduction in luminance of an outputted light in a desired direction.
Each of JP(A)-1-244490 and JP(A)-1-252933 presents a surface light source device which includes an outputted or emitted light adjustment member or a light diffusion plate having a light reflection pattern matching a distribution of an inverse number of outputted light luminance on a light output surface of a light guide. In such surface light source devices, however, there is also a drawback that impossibility of reuse of a light reflected by the outputted light adjustment member or the light diffusion plate causes a light loss, leading to a reduction in luminance of an outputted light in a desired direction.
Further, JP(A)-2-84618 presents a surface light source device which includes a linear light source arranged so as to face a light input surface of a light guide, at least one of a light output surface and a back surface of the light guide being set as a mat finished surface, and a prism sheet placed on the light output surface. Such a surface light source device can obtain very high luminance, but it is not satisfactory in uniformity of luminance in the light output surface. Also, the mat finished surface is subtly controlled to somehow increase a uniformity ratio of luminance. However, there is still a problem in subtle reproducibility of the mat finished surface configuration. Additionally, in such a surface light source device, because a distribution of an outputted light (distribution in directions vertical and parallel to light input surface) is too wide (especially in the direction parallel to light input surface), as a surface light source device used for a portable electronic device, requirements of low power consumption and high luminance cannot be satisfied.
On the other hand, as a surface light source device for making uniform luminance of an outputted light and increasing luminance by reducing a light loss, there is a disclosure in JP(A)-8-40719. A light guide for a surface light source device according to the disclosed technology is characterized in that at least one side end face of a plate-like transparent body is set as a light input surface, a surface substantially orthogonal thereto is set as a light output surface, at least one of the light output surface and its back surface is constituted of many substantially spherical minute convex bodies, a ratio of a minute average curvature radius and an average period of a lens group of the convex bodies is 3 to 10, and a ratio of an average deviation of a distribution of the minute average curvature radius and the minute average curvature radius is 0.8 or lower. However, as the light guide is made thinner and a ratio of a length to its thickness is made larger, it is difficult to obtain uniform output characteristics of a light in the light output surface only by a structure where the surface is constituted of many substantially spherical minute convex bodies.
Additionally, in JP(A)-7-171228, there has been presented a technology for obtaining a surface light source having a narrow distribution of outputted light rays of a backlight and high light output normal luminance of a peak light by forming a special saw-tooth prism structure in a light guide (technology of output light control structure). This method is very effective means capable of realizing very high normal luminance in a narrow visual field only by the light guide without using any prism sheets. On the other hand, a uniformity ratio of an outputted light luminance distribution tends to be lost greatly. As in the case of this method, it is very difficult to obtain a high uniformity ratio of luminance based on formation of many micro concave and convex portions by a mat finished surface or other dot patterns, a distribution of shapes thereof or the like to add a special function to the light guide itself. Thus, it has been a great technical problem to simultaneously obtain a high uniformity ratio without losing the special functionality added to the light guide itself.
Furthermore, in the prism structure, a light traveling in a direction vertical to a direction of an elongated prism is directed to the normal direction of the light output surface. However, since a light traveling in a direction oblique to the elongated prism cannot be directed to the normal direction of the light output surface, the amount of this light becomes a loss. Thus, especially the surface light source device using the point light source has had a problem in terms of light use efficiency. As a light source for such a surface light source device, a compact LED light source low in power consumption has been used. Examples are a surface light source device using a direct-under lighting LED light source, which is similar to that described in JP(A)-8-32120, a surface light source device including an LED light source disposed on a light guide end face and a V-shaped groove formed on the light guide in a light traveling direction, which is similar to that described in JP(A)-7-270624, a surface light source device including an LED light source disposed on a light guide end face and a roughened light guide surface, which is similar to that described in JP(A)-8-18429, and a surface light source device including an LED light source disposed at a light guide corner and using a scattering light guide having a light diffusion material dispersed inside, which is similar to that described in JP(A)-7-320514. In these surface light source devices, however, there is a problem that because of a wide distribution of an outputted light, luminance per power consumption cannot be sufficiently increased and, because of a spot shape of a light source, only a portion before the light source is bright, causing uneven luminance as a whole. Additionally, JP(A)-11-329039 presents a surface light source device where triangular convex shape portions are arranged in a back surface of a light guide concentrically to a point light source and discretely with each other. However, in such a surface light source device, there is a problem that because of the discrete arrangement of the triangular convex shape portions, a light propagated through the light guide cannot be efficiently outputted in the normal direction of a light output surface.
An object of the present invention is to provide a light source device such as a surface light source device or a linear light source device capable of setting a luminance uniformity ratio of an outputted light very high in a light output surface even if the device is relatively thin and large in area.
Another object of the present invention is to provide a light source device such as a surface light source device or a linear light source device capable of obtaining a very high uniformity ratio of an outputted light in a light output surface without losing functionality even in the case of using a light guide having the special functionality regarding an outputted light control function.
Still another object of the present invention is to provide a thin and uniform surface light source device low in power consumption and high in luminance, which uses a point light source such as an LED and which is suitable especially for a portable electronic device such as a portable telephone or a portable information terminal.